Add this to your .htacces file.

RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^https?://(www\.)?example\.com/.*$ [NC]
RewriteRule \.(gif|jpg|jpeg|bmp|zip|rar|mp3|flv|swf|xml|png|css|pdf)$ - [F]